Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University's Woodrow Wilson School. His new book, "Arsenal of Democracy: The Politics of National Security -- From World War II to the War on Terrorism," will be published this fall by Basic Books. Zelizer writes widely about current events.

PRINCETON, New Jersey (CNN) -- In response to the growing pressure for an investigation into potential abuses by the CIA and former Bush administration officials, Republican Sen. John Cornyn warned: "This is high-risk stuff. Because if we chill the ability or the willingness of our intelligence operatives and others to get information that's necessary to protect America, there could be disastrous consequences."

But Cornyn has it wrong. What chills our national security operations is not the discovery of wrongdoing. Rather, what chills our national security operations is tolerating programs that undermine the credibility of our institutions. When Americans are asked to go to war or are warned of dangerous threats, they must be able to believe the people they are hearing from.

Following the most recent revelations about the CIA, we have reached a tipping point where it is becoming impossible to continue dismissing these allegations as part of the past.

The House Intelligence Committee has announced that it will begin an investigation into the CIA's plans for a covert assassination program to target al Qaeda operatives, including allegations that then-Vice President Dick Cheney instructed the agency to hide the operation from Congress.

Meanwhile, Attorney General Eric Holder has announced that he might appoint a special prosecutor to investigate the use of torture by the CIA. Congressional Democrats have suggested this might just be the start.

This will not be the first time that Congress and the executive branch have looked into intelligence activities. During the 1970s, Congress dealt with comparable revelations about the CIA between the 1950s and early 1970s.

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The scandal began with the publication of shocking articles in December 1974 by Seymour Hersh that documented illegal activities by the CIA, including assassination plots and warrantless domestic surveillance. Hersh's stories were based on findings in a lengthy internal CIA report called the "Family Jewels." When CIA Director William Colby met with President Ford on January 3, 1975, to present the findings, he said: "We have a 25-year-old-institution which has done some things it shouldn't have."

Multiple investigations began. Democratic Rep. Otis Pike of New York led an investigation. Ford established a commission under Vice President Nelson Rockefeller and implemented some reforms through an executive order that included a ban on assassinations.

But the most influential of the investigations took place under Sen. Frank Church, who convened a set of dramatic hearings into the CIA in the fall of 1975. Church, who was planning to run for president in 1976, headed the 11-person panel that brought these revelations to the public.

The committee revealed that there had been attempts to assassinate numerous leaders such as Fidel Castro of Cuba and Rafael Trujillo of the Dominican Republican. On the first day of the hearings, Church pulled out one of the CIA's poison dart guns, equipped with a telescopic sight, to open a discussion about how the agency had run an 18-year, multimillion dollar program to develop biological weapons.

A few days into the hearings, the committee revealed that the CIA had intercepted letters that came from overseas to influential politicians, including Richard Nixon and Hubert Humphrey. The committee also revealed that the FBI was responsible for at least 238 illegal burglaries against dissident groups between 1942 and 1968.

The Church Committee and the other investigations served a crucial role in 1970s America. They offered Americans, weary from Vietnam and distrustful of their government, an unprecedented look into how the CIA worked and the ways in which the agency abused its authority.

The committee's efforts resulted in important reforms, including the establishment of congressional intelligence committees and the Foreign Intelligence Surveillance Act, or FISA, which imposed constraints on federal agents that have once again become the subject of contention. Most important, the public cleansing of the past gave Americans a better sense of what their government was doing and in some ways allowed the agency to regain its legitimacy

The Church Committee was far from perfect. The committee was not run with the same kind of efficiency as Sen. Sam Ervin's Watergate committee in 1973. There were also many moments when legislators, including Church, were grandstanding for the cameras. Church's run for the presidency would fizzle during the 1976 Democratic primaries. And some of the reforms imposed restraints on intelligence gathering that made less sense as technology changed -- along with the nature of our adversaries.

But the 1970s investigations were nonetheless essential in the process of making America's national security system more accountable to the citizenry and checking the virtually unlimited immunity from democratic scrutiny that federal agents had come to enjoy.

President Obama has thus far tried to avoid an investigation on the grounds that he wants to focus on the future, not the past.

But Obama's formulation, just like Cornyn's, is wrong. The president must support these investigations. This is not just about investigating the past. If our national security institutions are unaccountable, they will not be able to command the kind of public credibility they need in coming years.

The opinions expressed in this commentary are solely those of Julian E. Zelizer.